I don't know, because I'm not a physician, but it could be due to the way that the penis forms. The penis forms in two halves, which then fuse together. This accounts for the line which runs along the underside of the penis. It can be more pronounced in others, but everyone's got it.

My best advice, talk to you GP about it if you're really worried.

And the sun sets in the skyYou're the apple of my eyeIf the bomb goes off againIn my brain or on the trainI hope that I'm with you'Cos I wouldn't know what to doIt means nothing